Recently, resistance variable memory elements, which include Programmable Conductive Random Access Memory (PCRAM), have been investigated for suitability as semi-volatile and non-volatile random access memory devices. A typical PCRAM device is disclosed in U.S. Pat. No. 6,348,365 to Moore and Gilton. In typical PCRAM devices, conductive material, such as silver, is incorporated into chalcogenide material. The resistance of the chalcogenide material can be programmed to stable higher resistance and lower resistance states. The unprogrammed PCRAM device is normally in a high resistance state. A write operation programs the PCRAM device to a lower resistance state by applying a voltage potential across the chalcogenide material.
The programmed lower resistance state can remain intact for an indefinite period, typically ranging from hours to weeks, after the voltage potentials are removed. The PCRAM device can be returned to its higher resistance state by applying a reverse voltage potential of about the same order of magnitude as used to write the element to the lower resistance state. Again, the higher resistance state is maintained in a semi-volatile manner once the voltage potential is removed. In this way, such a device can function as a resistance variable memory element having two resistance states, which can define two logic states.
A PCRAM device can incorporate a chalcogenide glass, for example comprising germanium selenide (GexSe100-x). The germanium selenide glass may also incorporate silver (Ag) or silver selenide (Ag2Se).
The amorphous nature of the chalcogenide glass material used in a PCRAM device has a direct bearing on its programming characteristics. Thus, the incorporation of silver into the chalcogenide glass requires precise control of the glass composition and silver concentration so as not to cause the chalcogenide glass to change from the desired amorphous state to a crystalline state.
Exemplary preferred resistance variable devices are described in U.S. patent application Ser. No. 10/819,315 filed on Apr. 7, 2004, entitled “Layered Resistance Variable Memory Device and Method of Fabrication”, naming Kristy A. Campbell, Jiutao Li, Allen McTeer and John T. Moore as inventors. Certain exemplary preferred embodiments of such application disclose Ag received over GeSe received over Ag2Se as a composite of some of the switchable resistance variable material received intermediate a pair of electrodes in a memory device. In certain instances, however, it may be desirable to form Ag directly on (with “on” in this document meaning in at least some direct physical touching contact) Ag2Se. The preferred manner of depositing silver in the fabrication of such devices is by sputtering,from a silver target. Unfortunately when sputtering silver directly onto a silver selenide surface, the silver tends to agglomerate providing discontinuous and otherwise less than complete covering of the silver over the Ag2Se, even at deposition thicknesses on the magnitude of 2,000 Angstroms. Such silver agglomeration can cause subsequent processing problems during operation of such a memory cell. Use of a chalcogenide glass layer such as germanium selenide between the silver layer and the silver selenide tends to prevent such undesired silver agglomeration.
A typical resistance variable element comprises a pair of conductive electrodes having the chalcogenide glass material received therebetween. One such manner of forming a first of the conductive electrodes is to provide an elemental tungsten plug within an opening formed in electrically insulative material. The plug is typically formed by overfilling the opening with tungsten, followed by chemically mechanical polishing the tungsten back to at least the outermost surface of the insulative material to form an isolated tungsten plug within the opening. A chalcogenide glass layer is deposited thereover, followed by an outer electrode of the device for example also comprising elemental tungsten. The outer electrode and chalcogenide glass material are typically patterned to form a desired configuration of an individual resistance variable memory element.
Unfortunately, the formation of the typical elemental tungsten metal plug electrode of such devices by chemical mechanical polishing can produce less than optimized devices. For example, in some instances the chemical mechanical polishing typically causes the tungsten plugs to recess relative to an outermost surface of the surrounding insulating material after such polishing from about 40 Angstroms to about 80 Angstroms. Further, the tungsten as-deposited typically has one or both of cracks/crevices and sealed voids (sometimes referred to as “keyholes”). Also, the outermost surface of the tungsten within the openings after the polishing is significantly roughened, for example to a typical roughness of at least 5.0 nanometers RMS roughness. Any one or combination of these artifacts can make it difficult to get complete filling of the remaining volume of the openings with the subsequently deposited chalcogenide glass material of a resistance variable element, leading to less than optimum devices.
While the invention was motivated in addressing the above-identified issues, it is in no way so limited. For example, and by way of example only, the invention has applicability to the fabrication of conductive metal plugs in electrically insulative material, regardless of whether such are incorporated in resistance variable elements. The invention is only limited by the accompanying claims as literally worded, without interpretative or other limiting reference to the specification, and in accordance with the doctrine of equivalents.